In recent years, there have been proposed and developed various automotive steer-by-wire (SBW) vehicle steering systems in which a steering reaction torque applied to a steering wheel and a steer angle at each steered road wheel can be arbitrarily determined. Such an automotive SBW vehicle steering apparatus generally employs a pair of steering actuators, each mechanically disconnected from a steering wheel. One such SBW vehicle steering apparatus, which is configured to steer a plurality of steered road wheels independently of each other by means of respective steering actuators, has been disclosed in Japanese Patent Provisional Publication No. 2007-55409 (hereinafter is referred to as “JP2007-055409”), corresponding to United States Patent Application Publication No. US 2007/0045036 A1 and also disclosed in Japanese Patent Provisional Publication No. 2007-1564 (hereinafter is referred to as “JP2007-001564”). In a steering device (or a turning device including a turning motor and a speed reducer) disclosed in JP2007-055409, in order to avoid undesired interference between a steering actuator (or a turning actuator) and a hub carrier (a road-wheel side component part provided between upper and lower control arms), the turning actuator is laid out, such that the rotation axis of the turning actuator and the kingpin axis (serving as a turning center of the steered road wheel) are arranged substantially coaxial with each other. On the other hand, in a steer-by-wire system disclosed in JP2007-001564, in order to enhance the layout flexibility of engine component parts by compactifying the SBW system, a steering actuator is fixed onto or mounted on the vehicle body. The output (i.e., rotary motion) of the steering actuator is converted into oscillating motion of a pitman arm. The oscillating motion of the pitman arm is further transmitted through a linkage, such as a tie rod and a steering knuckle arm, so as to steer or turn the associated steered road wheel.